As is known, many pourable food products, such as fruit juice, UHT (ultra-high-temperature treated) milk, wine, tomato sauce, etc., are sold in packages made of sterilized packaging material.
A typical example of this type of package is the parallelepiped-shaped package for liquid or pourable food products known as Tetra Brik Aseptic (registered trademark), which is made by folding and sealing laminated strip packaging material.
The packaging material has a multilayer structure substantially comprising a base layer for stiffness and strength, which may comprise a layer of fibrous material, e.g. paper, or mineral-filled polypropylene material, and a number of lamination layers of heat-seal plastic material, e.g. polyethylene films, covering both sides of the base layer.
In the case of aseptic packages for long-storage products, such as UHT milk, the packaging material also comprises a layer of gas-barrier material, e.g. aluminium foil or ethyl vinyl alcohol (EVOH) film, which is superimposed on a layer of heat-seal plastic material, and is in turn covered with another layer of heat-seal plastic material forming the inner face of the package eventually contacting the food product.
Packages of this sort are normally produced on fully automatic packaging machines, on which a continuous tube is formed from the web-fed packaging material; the web of packaging material is sterilized on the packaging machine, e.g. by applying a chemical sterilizing agent, such as a hydrogen peroxide solution, which, once sterilization is completed, is removed from the surfaces of the packaging material, e.g. evaporated by heating; and the web of packaging material so sterilized is maintained in a closed, sterile environment, and is folded and sealed longitudinally to form a vertical tube.
The tube is filled with the sterilized or sterile-processed food product, and is sealed and subsequently cut along equally spaced cross sections to form pillow packs, which are then folded mechanically to form respective finished, e.g. substantially parallelepiped-shaped, packages.
Alternatively, the packaging material may be cut into blanks, which are formed into packages on forming spindles, and the packages are filled with the food product and sealed. One example of this type of package is the so-called “gable-top” package known by the trade name Tetra Rex (registered trademark).
To open the packages described above, various solutions of opening devices have been proposed.
A first solution of opening device comprises a patch defined by a small sheet of a heat-seal plastic material, and which is heat sealed over a respective hole on the side of the web eventually forming the inside of the package; and a pull-off tab applied to the opposite side of the packaging material and heat sealed to the patch. The tab and patch adhere to each other, so that, when the tab is pulled off, the portion of the patch heat sealed to it is also removed to uncover the hole.
Alternatively, a second solution of the opening devices comprises closable opening devices which are applied by injecting plastic material directly onto the holes of the web. In this case, the application station is a molding station.
Finally, a third solution of opening device comprises a frame defining an opening and fitted about a pierceable or removable portion of the packaging material.
The pierceable portion of the package may be defined by a so-called “prelaminated” hole, i.e. a hole formed in the base layer only and covered by the other lamination layers, including the layer of gas-barrier material. Also in this case, the application station is a molding station.
More precisely, the web is provided with a plurality of prelaminated holes in a packaging material factory and then fed to the packaging machine.
The web is then wound off from a reel within the packaging machine. Subsequently, the web is stepwise fed to the application station before the packaging material is folded to form a tube. In particular, the web is fed towards the molding station along an advancing direction.
The molding of opening devices at the molding station requires that pre-laminated holes are arrested in respective desired positions relative to the molding station.
In particular, the desired position is required for a correct molding of the opening device at the molding station.
EP-A-2357138, in the name of the same Applicant, discloses a unit for applying opening devices onto respective pre-laminated holes, substantially comprising:                a tensioning device for establishing a correct level of tension in the web of packaging material with the pre-laminated holes, which advances along an advancing direction;        the molding station, which is stepwise fed with the web by the feeding device and is adapted to injection mould a plurality of opening devices onto the web and in correspondence of respective pre-laminated holes of the web; and        an advancing device, which is arranged downstream of the molding station according to the advancing sense of the web along the advancing direction and adapted to advance the web along the advancing direction.        
In particular, the advancing device stepwise feeds one after the other and along the advancing direction a plurality of portions of the web each comprising three opening devices towards the molding station.
The molding station comprises a plurality, three in the known solution, of moulds, which inject the plastic material forming the opening devices onto the web and in correspondence of respective pre-laminated holes.
Furthermore, the pre-laminated holes are associated to respective magnetic markers.
In order to adjust the position of the pre-laminated holes with respect to relative moulds, the unit comprises a magnetic sensor for detecting the presence of markers while the web is advancing and generating respective measure signals associated to the real positions of the pre-laminated holes.
Still more precisely, the additional displacement along the advancing direction is associated to the difference between the detected position and the desired position of only one, namely the intermediate one, pre-laminated hole.
Even if the previously described known solution efficiently adjusts the position of the pre-laminated holes relative to the moulds, an increase in the number of the moulds remains highly desirable, in order to correspondingly increase the rate of application of the opening devices.
However, the known solution allows to correctly positioning in the desired position along the advancing direction only one reference pre-laminated hole with respect to relative mould, in particular the intermediate pre-laminated hole.
The remaining pre-laminated holes will not be arranged in the respective desired positions with respect to relative moulds. This is because, there are inevitable tolerance errors in the distance between homologous points, e.g. the axes, of the remaining pre-laminated holes and of the reference pre-laminated hole.
Unfortunately, the higher are the number of the moulds, the longer is the tolerance chain formed by the inevitable errors in the distances between homologous points, e.g. between the axes, of the reference pre-laminated hole and the remaining pre-laminated holes.
Accordingly, in the known solution, an increase in the number of moulds inevitably affects the precision in the positioning of the remaining pre-laminated holes with respect to the corresponding desired position and therefore to the respective moulds.
A need is therefore felt within the industry to increase the number of moulds without lengthen the tolerance chain formed by the inevitable errors in the distances between homologous points of the pre-laminated holes and, therefore, without affecting the precision in the final positioning of the pre-laminated holes with respect to the corresponding desired positions, and therefore, to the respective moulds.
Still in broader terms, a need is felt within the industry, when a plurality of operations must be carried out at desired positions onto respective areas of a web of packaging material, to increase the number of tools which carry out the operations, without lengthen the tolerance chain formed by the inevitable errors in the distances between homologous points of two areas and, therefore, without affecting the precision in the final positioning with respect to the corresponding desired position and, therefore, with respect to the tools.